1. Field of the Invention
The invention relates to servo-control systems which, more particularly, effectuate a smooth bumpless transition while switching from one transducer feedback signal to another.
2. Background of the Invention
During the last decade, extensive use has been made of servo-mechanisms in order to provide accurate control over some form of mechanical motion. These servo-mechanisms are typically comprised of a servo-control system, an actuator, and a feedback transducer. The actuator, typically electric, hydraulic or pneumatic, provides the desired motion, e.g., rotation or displacement, to drive a mechanical system. This motion is measured by the transducer, which generates a signal that is usually proportional to the value of the variable being measured. The signal provided by the transducer is fed back as an input to the servo-control system. Within the servo-control system, the feedback signal is subtracted from a command, or program, signal. A resulting difference signal is applied to a servo-amplifier which, in turn, drives the actuator. By virtue of the negative feedback inherent in this system, the actuator is driven such that the value of the transducer feedback signal will tend to equal the command signal thereby producing a zero-valued difference, or error, signal.
Oftentimes in such servo-mechanisms, the actuator or prime mover is an hydraulic cylinder, which is controlled by a very fast, sensitive hydraulic valve. The motion produced by the cylinder is usually characterized by a number of different variables, such as stroke, i.e., displacement of the cylinder from a known position, and--from the perspective of the object upon which the cylinder acts--longitudinal strain, cross-wise strain, force and stress. A signal proportional to each variable is simultaneously produced by a separate corresponding transducer and is available as a separate feedback signal. Typically, however, at any one time, the servo-mechanism is controlled by only one of the transducer feedback signals.
While the servo-mechanism is operating, it is often useful to switch between different transducer feedback signals, in order to change the type, or mode, of control from, for example, stroke control to true stress control. This requires switching between different feedback signals, i.e. the current and the desired feedback signals, which are produced by two different transducers. Since, at any instant, each and every transducer usually produces a signal having a value that is different from that produced by any other transducer, a difference exists between the current and desired feedback signals produced by the current and desired transducers, respectively. Consequently, at the instant a switchover between transducers is effectuated, the difference is imparted to the error signal. Consequently, this difference is applied to the servo-amplifier and thus causes a sudden "bump" or shift in the movement produced by the actuator. If this difference is sufficiently large, then the resulting large movement may disadvantageously damage or destroy the mechanical system being controlled.
Various attempts have been made in the art to provide smooth "bumpless" switching between a plurality of different feedback signals. However, these attempts, typified by that disclosed in U.S. Pat. No. 3,742,324 (issued to A. E. Gross et al on June 26, 1973), possess several drawbacks. Specifically, this patent discloses the concept of using a track and hold device to temporarily store the value that the error signal has immediately prior to a switchover. This stored error signal value is used to establish open loop control of the servo-mechanism during the switchover. Simultaneously therewith, the servo-control system monitors the feedback signal produced by the desired transducer and adjusts the command signal, by slewing it to a new value, such that the new value of the command signal less the signal produced by the desired transducer equals the value of the error signal stored in the track and hold device. At the instant equality occurs, the track and hold device is placed in the "track" mode thereby placing the desired transducer as the feedback element in a closed control loop. While this approach does produce a "bumpless" transition, it requires that the servo-control system both monitor the signal produced by the desired transducer and slew the command signal accordingly. Consequently, this requirement significantly increases the complexity of the circuitry of the servo-controller. Also this requirement disadvantageously injects a delay into the response of the servo-control system, which, in turn, lowers the overall response of the entire servo-mechanism.